Conductive adhesive

ABSTRACT

The conductive adhesive includes a thermosetting resin (A) and a conductive filler (B), and has a loss modulus at 200° C. from 5.0×10 4  Pa to 4.0×10 5  Pa.

TECHNICAL FIELD

The present disclosure relates to a conductive adhesive.

BACKGROUND ART

A conductive adhesive is often used for a flexible printed wiring board.For example, a flexible printed wiring board has been known to includean electromagnetic-wave shielding film bonded thereto and including ashielding layer and a conductive adhesive layer. In this case, theconductive adhesive needs to firmly bond an insulating film (a coverlay) provided on the surface of the flexible printed wiring board and ametal reinforcing plate together, and to ensure good conduction with aground circuit exposed from an opening of the insulating film.

In recent years, as a result of reduced electrical device size, therehas been a need to fill a small opening with a conductive adhesive toallow a ground circuit to be reliably conductive. For this reason,consideration has been made to improve the filling performance of theconductive adhesive (see, for example, Patent Document 1).

CITATION LIST Patent Document

[Patent Document 1] International Publication No. 2014/010524

SUMMARY OF THE INVENTION Technical Problem

However, the conductive adhesive has needed to have not only fillingperformance but also adhesiveness. Factors for improving the fillingperformance have not been sufficiently analyzed. If the composition ofthe conductive adhesive is changed to improve the adhesiveness,consideration needs to be again made to improve the filling performance.

It is an object of the present disclosure to clarify factors affectingthe filling performance of a thermosetting conductive adhesive and toimprove the filling performance and adhesiveness of a conductiveadhesive having a wide range of composition.

Solution to the Problem

A conductive adhesive according to an aspect of the present disclosureincludes a thermosetting resin (A); and a conductive filler (B), whereinthe conductive adhesive has a loss modulus at 200° C. from 5.0×104 Pa to4.0×105 Pa.

In conductive adhesive according to the aspect, the conductive adhesivemay contain from 40 to 140 parts by mass of the loss modulus modifier(C) relative to 100 parts by mass of the thermosetting resin (A).s

Advantages of the Invention

According to the conductive adhesive of the present disclosure, thefilling performance and adhesiveness of the conductive adhesive having awide range of composition can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of anelectromagnetic-wave shielding film.

FIG. 2 is a cross-sectional view illustrating a variation of anelectromagnetic-wave shielding film.

FIGS. 3A to 3C are cross-sectional views sequentially illustrating amethod for forming an electromagnetic-wave shielding film.

FIG. 4 is a cross-sectional view illustrating a shield printed wiringboard.

FIG. 5 is a cross-sectional view illustrating a process for forming ashield printed wiring board.

FIG. 6 is a cross-sectional view illustrating a process for bonding aconductive reinforcing plate.

FIG. 7 is a cross-sectional view illustrating another process forbonding the conductive reinforcing plate.

FIG. 8 is a cross-sectional view illustrating still another process forbonding the conductive reinforcing plate.

FIG. 9 illustrates a method for measuring interconnect resistance.

FIG. 10 is a graph showing the dependence of loss modulus ontemperature.

FIG. 11 is a graph showing the dependence of storage modulus ontemperature.

DETAILED DESCRIPTION

A conductive adhesive of this embodiment contains a thermosetting resin(A) and a conductive filler (B), and has a loss modulus at 200° C. from8×10⁴ Pa to 5×10⁵ Pa.

The present inventors' active studies showed that the conductiveadhesive maintains its loss modulus within a somewhat high loss modulusrange in a range of temperatures from the embedding temperature at whichthe conductive adhesive is embedded to the reflow temperature, thusimproving the filling performance and adhesiveness. If the loss modulusof the conductive adhesive is significantly reduced at the embeddingtemperature, the conductive adhesive absorbs the pressing pressure, thusmaking it less likely for the conductive adhesive to be pushed into aconnection hole. On the other hand, if the loss modulus of theconductive adhesive at the embedding temperature is excessively high,the conductive adhesive is less likely to be deformed, and to be pushedinto the connection hole.

The highest temperature to which the conductive adhesive is exposed whenbonded may be about 130° C. to 190° C., and the reflow temperature maybe about 240° C. to 260° C. For this reason, the loss modulus at 140° C.may be preferably 5.0×10⁴ Pa or more and more preferably 7.0×10⁴ Pa ormore, and may be preferably 3.0×10⁵ Pa or less and more preferably2.5×10⁵ Pa or less. The loss modulus at 170° C. may be preferably5.0×10⁴ Pa or more and more preferably 7.0×10⁴ Pa or more, and may bepreferably 4.0×10⁵ Pa or less and more preferably 3.5×10⁵ Pa or less.Further, the loss modulus at 200° C. may be preferably 5.0×10⁴ Pa ormore and more preferably 7.0×10⁵ Pa or more, and may be preferably4.0×10⁵ Pa or less and more preferably 3.5×10⁵ Pa or less.

This allows the conductive adhesive to maintain its sufficientelasticity when the conductive adhesive is to be embedded. Note that theloss modulus can be measured by a method shown in examples.

—Thermosetting Resin (A)—

The first resin component (A1) of the thermosetting resin (A) includesmolecules each having two or more first functional groups. The firstfunctional groups may be any functional group that reacts with thesecond functional group included in the second resin component (A2), butmay be, for example, an epoxy group, an amide group, a hydroxyl group,or any other functional group. In particular, an epoxy group isrecommended.

If the first functional group is an epoxy group, examples of the firstresin component (A1) include a bisphenol epoxy resin (such as abisphenol A epoxy resin, a bisphenol F epoxy resin, or a bisphenol Sepoxy resin), a glycidyl ether epoxy resin (such as a spirocyclic epoxyresin, a naphthalene epoxy resin, a biphenyl epoxy resin, a terpeneepoxy resin, a tris(glycidyloxyphenyl)methane, or atetrakis(glycidyloxyphenyl)ethane), a glycidyl amine epoxy resin (suchas a tetraglycidyl diamino diphenylmetane), a tetrabrombisphenol A epoxyresin, a novolac epoxy resin (such as a cresol novolac epoxy resin, aphenol novolac epoxy resin, an α-naphthol novolac epoxy resin, or abrominated phenol novolac epoxy resin), and a rubber-modified epoxyresin. One kind of these substances may be used alone, or two or morekinds thereof may be used in combination. These epoxy resins may besolid or liquid at room temperature. A situation where the epoxy resinsare “solid at room temperature” means a situation where the epoxy resinsdo not have flowability at a temperature of 25° C. without a solvent. Asituation where the epoxy resins are “liquid at room temperature” meansa situation where the epoxy resins have flowability under the sameconditions.

The number average molecular weight of the first resin component (A1) isnot specifically limited. However, to increase the bulk strength of theadhesive, it is preferably 500 or more and more preferably 1,000 ormore. In addition, to increase adhesiveness, the number averagemolecular weight is preferably 10,000 or less and more preferably 5,000or less.

The second resin component (A2) has a second functional group thatreacts with the first functional group of the first resin component(A1). If the first functional group is an epoxy group, the secondfunctional group may be a hydroxyl group, a carboxyl group, an epoxygroup, an amino group, or any other functional group. In particular, ahydroxyl group and a carboxyl group are recommended. If the firstfunctional group is an amide group, the second functional group may be ahydroxyl group, a carboxyl group, an amino group, or any otherfunctional group. If the first functional group is a hydroxyl group, thesecond functional group may be an epoxy group, a carboxyl group, anamide group, or any other functional group.

If the second functional group is a carboxyl group, the second resincomponent (A2) may be, for example, an urethane-modified polyesterresin. The urethane-modified polyester resin is a polyester resincontaining an urethane resin as a copolymer component. Theurethane-modified polyester resin can be obtained through a reactionbetween an isocyanate component and a terminal hydroxyl group of apolyester resin that has been obtained through condensationpolymerization of an acid component, such as a polyvalent carboxylicacid or an anhydride thereof, and a glycol component. In addition, anurethane-modified polyester resin can be obtained through a simultaneousreaction among an acid component, a glycol component, and an isocyanatecomponent.

Non-limiting examples of the acid component include terephthalic acid,isophthalic acid, orthophthalic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 4,4′-diphenylcarboxylic acid,2,2′-diphenyldicarboxylic acid, 4,4′-diphenyletherdicarboxylic acid,adipic acid, azelaic acid, sebacic acid, 1,4-cyclohexanedicarboxylicacid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylicacid, 4-methyl-1,2-cyclohexanedicarboxylic acid, dimer acid, dehydratedtrimellitic acid, dehydrated pyromellitic acid, and5-(2,5-dioxotetrahydro-3-furanyl)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride.

Non-limiting examples of the glycol component include dihydric alcohols(such as ethylene glycol, propylene glycol, 1,3-propanediol,2-methyl-1,3-propanediol, 1,2-butanediol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,3-methyl-1,5-pentanediol, neopentylglycol, diethylene glycol,dipropylene glycol, 2,2,4-trimethyl-1,5-pentanediol, cyclohexanedimethanol, neopentyl hydroxypivalate, an ethylene oxide adduct and apropylene oxide adduct of bisphenol A, an ethylene oxide adduct and apropylene oxide adduct of hydrobisphenol A, 1,9-nonanediol, 2-methyloctanediol, 1,10-decanediol, 2-butyl-2-ethyl-1,3-propanediol,tricyclodecane dimethanol, polyethylene glycol, polypropylene glycol,and polytetramethylene glycol), and, if necessary, tri- or more valentpolyvalent alcohols (such as trimethyl propane, trimethylol ethane, andpentaerythritol).

Non-limiting examples of the isocyanate component include 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, p-phenylene isocyanate,diphenyl methane diisocyanate, m-phenylene diisocyanate, hexamethylenediisocyanate, tetramethylene diisocyanate,3,3′-dimethoxy-4,4′-biphenylene diisocyanate, 1,5-naphthalenediisocyanate, 2,6-naphthalene diisocyanate,3,3′-dimethoxy-4,4′-diisocyanate, 4,4′-diphenylene diisocyanate,1,5-xylylene diisocyanate, 1,3-methylcyclohexane diisocyanate,1,4-methylcyclohexane diisocyanate, and isophorone diisocyanate.

The second resin component (A2) is not limited to an urethane-modifiedpolyester resin. Examples of the second resin component (A2) include anacid anhydride-modified polyester resin and an epoxy resin.

If an urethane-modified polyester resin is used as the second resincomponent (A2), the second resin component (A2) has a glass transitiontemperature of preferably 5° C. or higher, more preferably 10° C. orhigher, and still more preferably 30° C. or higher to further improvethe filling performance. The glass transition temperature is preferably100° C. or lower, more preferably 90° C. or lower, and still morepreferably 80° C. or lower. Note that the glass transition temperaturecan be measured with a differential scanning calorimeter (DSC).

If an urethane-modified polyester resin is used as the second resincomponent (A2), the second resin component (A2) has a number averagemolecular weight (Mn) of preferably 10,000 or more and 50,000 or lessand more preferably 30,000 or less to further improve the fillingperformance. Note that Mn can be a value measured by gel permeationchromatography (GPC) and calculated in terms of styrene.

If an urethane-modified polyester resin having a carboxyl group is usedas the second resin component (A2), the second resin component (A2) hasan acid value of preferably 5 mgKOH/g or more, more preferably 10mgKOH/g or more, and still more preferably 15 mgKOH/g or more, toincrease heat resistance. The acid value is preferably 50 mgKOH/g orless, more preferably 45 mgKOH/g or less, and still more preferably 40mgKOH/g or less. Note that the acid value can be measured in accordancewith JIS K0070:1992.

If the first resin component (A1) is an epoxy resin, and the secondresin component (A2) is an urethane-modified polyester resin, the ratio(A1/(A1+A2)) of the mass of the first resin component (A1) to the totalmass of the first resin component (A1) and the second resin component(A2) may be 1% by mass or more, preferably 3% by mass or more, and 15%by mass or less, preferably 10% by mass or less. Setting the mass ratioof the first resin component (A1) and the second resin component (A2) tosuch a range can improve adhesiveness.

The thermosetting resin (A) can contain a curing agent (A3) thataccelerates a reaction between the first functional group of the firstresin component (A1) and the second functional group of the second resincomponent (A2). The curing agent (A3) can be appropriately selected inaccordance with the kinds of the first and second functional groups. Ifthe first functional group is an epoxy group, and the second functionalgroup is a hydroxyl group, examples of the curing agent include animidazole curing agent, a phenolic curing agent, and a cationic curingagent. One kind of these substances may be used alone, or two or morekinds thereof may be used in combination.

Examples of the imidazole curing agent include compounds in which analkyl group, an ethylcyano group, a hydroxyl group, an azine, etc., areadded to the imidazole ring, such as 2-phenyl-4,5-dihydroxymethylimidazole, 2-heptadecylimidazole, 2,4-diamino-6-(2′-undecylimidazolyl)ethyl-S-triazine, 1-cyanoethyl-2-phenylimidazole,2-phenylimidazole, 5-cyano-2-phenylimidazole,2,4-diamino-6-[2′methylimidazolyl-(1′)]-ethyl-S-triazine isocyanuricacid adduct, 2-phenyl imidazole isocyanuric acid adduct, 2-methylimidazole isocyanuric acid adduct, and1-cyanoethyl-2-phenyl-4,5-di(2-cyanoethoxy)methylimidazole.

Examples of the phenolic curing agent include novolac phenol and anaphtholic compound.

Examples of the cationic curing agent include an amine salt of borontrifluoride, an antimony pentachloride-acetyl chloride complex, asulfonium salt having a phenethyl group or an allyl group.

The curing agent (A3) does not have to be blended. However, toaccelerate the reaction, the content of the curing agent (A3) withrespect to 100 parts by mass of the first resin component (A1) ispreferably 0.3 or more parts by mass and more preferably 1 or more partsby mass, and is preferably 20 or less parts by mass and more preferably15 or less parts by mass.

—Conductive Filler (B)—

Non-limiting examples of the conductive filler (B) include a metalfiller, a metal-sheathed resin filler, a carbon filler, and a mixture ofthese fillers. Examples of the metal filler include a copper powder, asilver powder, a nickel powder, a silver-coated copper powder, agold-coated copper powder, a silver-coated nickel powder, and agold-coated nickel powder. These metal powders can be obtained throughelectrolysis, atomization, reduction, or any other process. Inparticular, any one of the silver powder, the silver-coated copperpowder, and the copper powder is recommended.

To improve contact between filler powders, the average particle size ofthe conductive filler (B) is, but not limited to, preferably 1 μm ormore and more preferably 3 μm or more, and preferably 50 μm or less andmore preferably 40 μm or less. The shape of the conductive filler (B)may be, but not limited to, a spherical shape, a flaky shape, adendritic shape, a fibrous shape, or any other shape. However, to obtaina good interconnect resistance, the shape is preferably dendritic.

The content of the conductive filler (B) relative to the total solidcontent can be appropriately selected depending on the intended use, butis preferably 5% by mass or more and more preferably 10% by mass ormore, and is preferably 95% by mass or less and more preferably 90% bymass or less. To improve the filling performance, the content ispreferably 70% by mass or less and more preferably 60% by mass or less.If anisotropic conductivity is to be achieved, the content is preferably40% by mass or less and more preferably 35% by mass or less.

—Optional Components—

The conductive adhesive according to this embodiment can contain, asoptional components, an anti-foam agent, an antioxidant, a viscositymodifier, a diluent, an anti-settling agent, a leveling agent, acoupling agent, a coloring agent, a fire retardant, and any other agent.

(Modification of Loss Modulus)

The loss modulus of the conductive adhesive can be modified, forexample, by adding a loss modulus modifier (C) to the conductiveadhesive. Examples of the loss modulus modifier (C) include an organicsalt, talc, carbon black, and silica. One kind of these materials may beused alone, or two or more kinds thereof may be used together. Such aprocess makes it possible to set the loss modulus at about 140° C. to200° C. to be a predetermined value.

The organic salt is not specifically limited. However, phosphates suchas polyphosphates and metal salts of phosphinic acid are recommended,and metal salts of phosphinic acid are further recommended. Amongorganic salts, phosphates such as polyphosphates and metal salts ofphosphinic acid are recommended, and metal salts of phosphinic acid arefurther recommended. Examples of the metal salts of phosphinic acidinclude an aluminum salt, a sodium salt, a potassium salt, a magnesiumsalt, and a calcium salt. In particular, an aluminum salt isrecommended. Examples of the polyphosphates include a melamine salt, amethylamine salt, an ethylamine salt, a diethylamine salt, atriethylamine salt, an ethylenediamine salt, a piperazine salt, apyridine salt, a triazine salt, and an ammonium salt. In particular, amelamine salt is recommended. Examples of the organic salts other thanphosphates include melamine cyanurate, melamine pyrophosphate, and melammethanesulfonate. In particular, melamine cyanurate is recommended.

To bring the loss modulus within a predetermined range, the content ofthe loss modulus modifier (C), such as an organic salt, with respect to100 parts by mass of the thermosetting resin (A) is 40 or more parts bymass and more preferably 50 or more parts by mass, and is preferably 140or less parts by mass, more preferably 120 or less parts by mass, stillmore preferably 100 or less parts by mass, and yet more preferably 80 orless parts by mass.

Preferably, the loss modulus modifier (C), such as an organic salt, doesnot project from the film of the conductive adhesive, and has a particlesize that is smaller than the thickness of the film of the conductiveadhesive. Depending on the thickness of the film to be finally formed,the median size of the organic salt may be preferably 5 μm or less andmore preferably 4 μm or less. For easy handling, the median size may bepreferably 1 μm or more and more preferably 2 μm or more. Note that themedian size is a particle size at which the cumulative frequency fromthe smallest particle size is 50% on a particle-size distribution curveobtained with a laser diffraction particle size distribution analyzer(where the ordinate represents the cumulative frequency (%), and theabscissa represents the particle size).

(Electromagnetic-Wave Shielding Film)

The conductive adhesive of the present disclosure can be used for anelectromagnetic-wave shielding film 101 including a protective layer 112and a conductive adhesive layer 111 as shown in FIG. 1. Such anelectromagnetic-wave shielding film allows the conductive adhesive layer111 to function as a shield. As shown in FIG. 2, a shielding layer 113may be separately provided between the protective layer 112 and theconductive adhesive layer 111.

—Protective Layer—

The protective layer 112 may be any layer that has sufficient insulatingperformance and protects the conductive adhesive layer 111 and, ifnecessary, the shielding layer 113. Examples of the protective layer 112include a thermoplastic resin composition, a thermosetting resincomposition, and an active energy ray-curable composition.

Non-limiting examples of the thermoplastic resin composition include astyrene resin composition, a vinyl acetate resin composition, apolyester resin composition, a polyethylene resin composition, apolypropylene resin composition, an imide resin composition, and anacrylic resin composition. Non-limiting examples of the thermosettingresin composition include a phenolic resin composition, an epoxy resincomposition, an urethane resin composition, a melamine resincomposition, and an alkyd resin composition. Non-limiting examples ofthe active energy ray-curable composition include a polymerizablecompound including molecules each having at least two (meth)acryloyloxygroups. The protective layer may be made of a single material, or may bemade of two or more materials.

The protective layer 112 may be a stack of two or more layers made ofdifferent materials or having different physical properties such as thehardness or the elasticity modulus. For example, if a stack of an outerlayer having a low hardness and an inner layer having a high hardness isused, the outer layer functions as a cushion. This can reduce thepressure applied to the shielding layer 113 in the step of bonding theelectromagnetic-wave shielding film 101 to the printed wiring boardunder heating and pressure. This can substantially prevent the shieldinglayer 113 from being broken due to a step formed on the printed wiringboard.

The protective layer 112 may contain, as necessary, at least one of acuring promoter, a tackifier, an antioxidant, a pigment, a colorant, aplasticizer, an ultraviolet absorber, an anti-foam agent, a levelingagent, a filler, a fire retardant, a viscosity modifier, an antiblockingagent, and other agents.

The thickness of the protective layer 112 is not specifically limited,and can be appropriately determined as necessary. The thickness of theprotective layer 112 may be preferably 1 μm or more and more preferably4 μm or more, and may be preferably 20 μm or less, more preferably 10 μmor less, and still more preferably 5 μm or less. Setting the thicknessof the protective layer 112 to be 1 μm or more allows the conductiveadhesive layer 111 and the shielding layer 113 to be adequatelyprotected. Setting the thickness of the protective layer 112 to be 20 μmor less allows the electromagnetic-wave shielding film 101 to besufficiently flexible, and facilitates using the electromagnetic-waveshielding film 101 for a member that needs to be flexible.

—Shielding Layer—

If the electromagnetic-wave shielding film 101 includes the shieldinglayer 113, the shielding layer 113 may be configured as a metal layer, aconductive filler, or any other layer. If the shielding layer 113 is ametal layer, the metal layer may be made of any one of nickel, copper,silver, tin, gold, palladium, aluminum, chromium, titanium, zinc, andany other metal, or an alloy containing two or more of these metals. Themetal layer can be produced by using metal foil or by depositing a metalfilm through an additive process. Examples of the additive processinclude electrolytic plating, electroless plating, sputtering,electron-beam evaporation, vacuum evaporation, chemical vapor deposition(CVD), and metal-organic chemical vapor deposition (MOCVD).

If the shielding layer 113 is configured as the conductive filler, metalnanoparticles or a conductive filler may be used. Examples of the metalnanoparticles include silver nanoparticles, gold nanoparticles, andother nanoparticles. Examples of the conductive filler include a metalfiller, a metal-sheathed resin filler, a carbon filler, and a mixture ofthese fillers. Examples of the metal fillers include a copper powder, asilver powder, a nickel powder, a silver-coated copper power, agold-coated copper powder, a silver-coated nickel powder, and agold-coated nickel powder. These metal powders can be obtained throughelectrolysis, atomization, and reduction. Examples of the shape of themetal powders include a spherical shape, a flaky shape, a fibrous shape,and a dendritic shape.

The metal material and thickness of the shielding layer 113 merely needsto be appropriately selected according to the requiredelectromagnetic-wave shielding effect and tolerance to repeated flexingand sliding. The thickness may be about 0.1 μm to 12 μm.

If the electromagnetic-wave shielding film 101 includes the shieldinglayer 113, the content of the conductive filler (B) in the conductiveadhesive to be the conductive adhesive layer 111 is preferably 3% bymass or more and more preferably 5% by mass or more, to improve theshielding performance. To reduce cost, the upper limit of the content ispreferably 40% by mass or less and more preferably 35% by mass or less.

If the electromagnetic-wave shielding film 101 does not include theshielding layer 113, and the conductive adhesive layer 111 functions asa shield, the content of the conductive filler (B) in the conductiveadhesive is preferably set to be 40% by mass or more, to providesufficient shielding performance. To increase tolerance of theelectromagnetic-wave shielding film 101 to flexing, the upper limit ofthe content of the conductive filler (b) is preferably 90% by mass orless.

(Method for Forming Electromagnetic-Wave Shielding Film)

An example of a method for forming an electromagnetic-wave shieldingfilm 101 will now be described. However, this method is merely anexample of the present disclosure.

—Formation of Protective Layer—

First, as shown in FIG. 3A, a protective layer composition is appliedonto a support substrate 151 to form a protective layer 112. Theprotective layer composition can be prepared through addition ofsuitable amounts of a solvent and other compounding ingredients to aresin composition. Examples of the solvent include toluene, acetone,methyl ethyl ketone, methanol, ethanol, propanol, and dimethyl formamideExamples of the other compounding ingredients that can be added includea crosslinker, a polymerization catalyst, a curing promoter, and acoloring agent. The other compounding ingredients may be added ifnecessary, and do not have to be added. The process of applying theprotective layer composition to the support substrate 151 is notspecifically limited. Examples of the process include known techniquessuch as lip coating, comma coating, gravure coating, and slot-diecoating.

The support substrate 151 may be, for example, in the form of a film.The support substrate 151 is not specifically limited. Examples of thematerial of the support substrate 151 include a polyolefinic material, apolyester material, a polyimide material, and a polyphenylene sulfidematerial. A release agent layer may be provided between the supportsubstrate 151 and the protective layer composition.

The protective layer composition that has been applied onto the supportsubstrate 151 is heated and dried to remove the solvent, thereby formingthe protective layer 112. The support substrate 151 can be separatedfrom the protective layer 112. The support substrate 151 can beseparated after the electromagnetic-wave shielding film 101 is bonded tothe printed wiring board. This allows the support substrate 151 toprotect the electromagnetic-wave shielding film 101.

—Formation of Shielding Layer—

Next, as shown in FIG. 3B, a shielding layer 113 is formed on thesurface of the protective layer 112. If the shielding layer 113 is ametal film, the shielding layer 113 can be formed through electrolyticplating, electroless plating, sputtering, electron-beam evaporation,vacuum evaporation, CVD, MOCVD, or any other process. If the shieldinglayer 113 is made of a conductive filler, a solvent containing theconductive filler can be applied onto the surface of the protectivelayer and then dried to form the shielding layer 113. If theelectromagnetic-wave shielding film 101 includes no shielding layer 113,this process step can be omitted.

—Formation of Conductive Adhesive Layer—

Next, as shown in FIG. 3C, a conductive adhesive layer composition isapplied onto the shielding layer 113, and then dried by heating toremove the solvent, thereby forming a conductive adhesive layer 111.

The conductive adhesive layer composition contains a conductive adhesiveand a solvent. Examples of the solvent include toluene, acetone, methylethyl ketone, methanol, ethanol, propanol, and dimethyl formamide. Theproportion of the conductive adhesive in the conductive adhesive layercomposition merely needs to be appropriately set in accordance with thethickness of the conductive adhesive layer 111 and other factors. Theprocess of applying the conductive adhesive layer composition onto theshielding layer 113 is not specifically limited. Examples of the processinclude lip coating, comma coating, gravure coating, and slot-diecoating.

The conductive adhesive layer 111 preferably has a thickness of 1 μm to50 μm. Setting the thickness to be 1 μm or more allows the conductiveadhesive layer to have sufficient filling performance and to beadequately connected to a ground circuit. Further, setting the thicknessto be 50 μm or less can satisfy the need to reduce the layer thickness,and helps reduce cost.

If necessary, a release substrate (a separate film) may be bonded to thesurface of the conductive adhesive layer 111. It is possible to use, asthe release substrate, a base film, made of polyethylene terephthalate,polyethylene naphthalate, or other materials, to which a silicone ornon-silicone release agent is applied on a surface where the conductiveadhesive layer 111 is formed. The thickness of the release substrate isnot specifically limited, and can be appropriately determined in view ofease of use.

(Shield Printed Wiring Board)

The electromagnetic-wave shielding film 101 according to this embodimentcan be used for a shield printed wiring board 103 shown in FIG. 4. Asshown in FIG. 4, the shield printed wiring board 103 includes a printedwiring board 102 and an electromagnetic-wave shielding film 101.

The printed circuit board 102 includes, for example, a base member 122and a printed circuit including a ground circuit 125 provided on thebase member 122. An insulating film 121 is bonded onto the base member122 through an adhesive layer 123. The insulating film 121 has anopening 128 through which the ground circuit 125 is exposed. A surfacelayer 126 that is a gold plated layer is provided on an exposed portionof the ground circuit 125. Note that the printed wiring board 102 may bea flexible substrate or a rigid substrate.

Now, a method for forming the shield printed wiring board 103 will bedescribed. First, as shown in FIG. 5, a printed wiring board 102 and anelectromagnetic-wave shielding film 101 are prepared.

Next, the electromagnetic-wave shielding film 101 is arranged on, andtemporarily fixed to, the printed wiring board 102 so that theconductive adhesive layer 111 is located over the opening 128. Then, theelectromagnetic-wave shielding film 101 and the printed wiring board 102are vertically sandwiched between two hot plates (not shown) heated to apredetermined temperature (for example, 120° C.) so as to be pressed ata predetermined pressure (for example, 0.5 MPa) for a short time (forexample, 5 seconds). As a result, the electromagnetic-wave shieldingfilm 101 is temporarily fastened to the printed wiring board 102.

Subsequently, the temperatures of two hot plates are set at apredetermined temperature (for example, 170° C.) higher than thoseduring the temporary fixing process. A predetermined pressure (forexample, 3 MPa) is applied to the electromagnetic-wave shielding film101 for a predetermined time (for example, 30 minutes) using these hotplates. As a result, the electromagnetic-wave shielding film 101 can befixed on the printed wiring board 102.

In this case, part of the conductive adhesive layer 111 which has beensoftened by the heat flows into the opening 128 due to the pressureapplied. As a result, the shielding layer 113 and the ground circuit 125are connected together. The part of the conductive adhesive layer 111that is large enough to fill the opening 128 is present between theshielding layer 113 and the printed wiring board 102. Thus, theelectromagnetic-wave shielding film 101 and the printed wiring board 102are bonded together with sufficient strength. The conductive adhesivelayer 111, which has high filling performance, can have high connectionstability even when exposed to high temperatures during reflowing in acomponent mounting process.

Thereafter, a solder reflow process for component mounting is performed.In the reflow process, the electromagnetic-wave shielding film 101 andthe printed wiring board 102 are exposed to a high temperature of about260° C. Non-limiting examples of components to be mounted include, inaddition to a connector and an integrated circuit, a chip component suchas a resistor and a capacitor.

The conductive adhesive of this embodiment, which has high fillingperformance, sufficiently fills the opening 128 even if the diameter ofthe opening 128 is small, such as 1 mm or less. This can ensure goodconnection between the electromagnetic-wave shielding film 101 and theground circuit 125. If the conductive adhesive insufficiently fills theopening, fine bubbles enter the gap between a combination of the surfacelayer 126 and the insulating film 121 and the conductive adhesive layer111. As a result, when the conductive adhesive layer 111 is exposed tohigh temperatures in the reflow process, the bubbles may grow, and theinterconnect resistance of the conductive adhesive layer 111 mayincrease. This may cause the conductive adhesive layer 111 to beseparated at worst. However, the conductive adhesive layer 111 of thisembodiment has good adhesiveness with the surface layer 126 and theinsulating film 121, and keeps having good adhesiveness even after thereflow process. Thus, low interconnect resistance can be maintained.

It is recommended that the interconnect resistance after the reflowprocess as an index of filling performance be as low as possible.However, the interconnect resistance to an opening with a diameter of0.5 mm to be described in examples, for example, can be set to bepreferably 300 mΩ/hole or less, more preferably 250 mΩ/hole or less, andstill more preferably 200 mΩ/hole or less.

To allow the conductive adhesive to have high filling performance, theconductive adhesive preferably maintains its loss modulus within asomewhat high loss modulus range in a range of temperatures from thehighest one (e.g., 170° C.) of temperatures to which the conductiveadhesive is exposed when bonded to the printed wiring board to thereflow temperature (e.g., 260° C.). Specifically, the loss modulus at140° C. may be preferably 5.0×10⁴ Pa or more and more preferably 7.0×10⁴Pa or more, and may be preferably 3.0×10⁵ Pa or less and more preferably2.5×10⁵ Pa or less. The loss modulus at 170° C. may be preferably5.0×10⁴ Pa or more and more preferably 7.0×10⁴ Pa or more, and may bepreferably 4.0×10⁵ Pa or less and more preferably 3.5×10⁵ Pa or less.Further, the loss modulus at 200° C. may be preferably 5.0×10⁴ Pa ormore and more preferably 7.0×10⁴ Pa or more, and may be preferably4.0×10⁵ Pa or less and more preferably 3.5×10⁵ Pa or less.

To allow the conductive adhesive of the present disclosure to have highfilling performance, the conductive adhesive preferably maintains itsloss modulus within a somewhat low storage modulus range in a range oftemperatures from the highest one (e.g., 170° C.) of temperatures towhich the conductive adhesive is exposed when bonded to the printedwiring board to the reflow temperature (e.g., 260° C.). Specifically,the storage modulus at 190° C. may be preferably 1.0×10⁵ Pa or more and2.0×10⁵ Pa or less. The storage modulus at 170° C. may be preferably1.0×10⁴ Pa or more and 2.5×10⁵ Pa or less. The storage modulus at 120°C. may be preferably 1×10⁵ Pa or more and 4.0×10⁵ Pa or less. Thestorage modulus at 70° C. may be preferably 1.0×10⁵ Pa or more and 1×10⁶Pa or less. Setting the storage modulus within the above range makes itdifficult to accumulate latent stress in the conductive adhesive duringhot pressing. As a result, the filling performance is improved.

Furthermore, preferably, the storage modulus curve of the conductiveadhesive of the present disclosure from 40° C. to 120° C. does not showa distinct maximum value greater than the storage modulus at 40° C. Asituation where the storage modulus curve does not show a maximum valuein this temperature range makes it difficult to accumulate latent stressin the conductive adhesive during hot pressing. As a result, the fillingperformance is improved.

To firmly bond the electromagnetic-wave shielding film 101, the peelstrength between the conductive adhesive layer 111 and the surface layer126 is preferably higher, and more specifically, is preferably 3.0 N ormore per 10 mm, more preferably 3.5 N or more per 10 mm, and still morepreferably 4.0 N or more per 10 mm. The peel strength between theconductive adhesive layer 111 and the insulating film 121 is preferablyhigher, and more specifically, is preferably 3.0 N or more per 10 mm,more preferably 5.0 N or more per 10 mm, and still more preferably 7.0 Nor more per 10 mm

The base member 122 may be, for example, a resin film or any other film,specifically, a film made of a resin such as polypropylene, cross-linkedpolyethylene, polyester, polybenzimidazole, polyimide, polyimideamide,polyetherimide, polyphenylene sulfide, or any other resin.

Non-limiting examples of the material of the insulating film 121 includeresins such as polyethylene terephthalate, polypropylene, cross-linkedpolyethylene, polyester, polybenzimidazole, polyimide, polyimideamide,polyetherimide, and polyphenylene sulfide. The thickness of theinsulating film 121 is not specifically limited, but may be about 10 μmto 30 μm.

The printed circuit including the ground circuit 125 may be, forexample, a copper wiring pattern formed on the base member 122. Thesurface layer 126 is not limited to a gold plated layer, and may be alayer made of copper, nickel, silver, tin, or any other metal. Note thatthe surface layer 126 may be provided if necessary, and may be preventedfrom being provided.

The conductive adhesive of this embodiment has good filling performanceand good adhesiveness, and is particularly effective at bonding theelectromagnetic-wave shielding film 101 to the printed wiring board 102.However, this conductive adhesive is also useful in other applicationsin which the conductive adhesive is used. For example, this conductiveadhesive can be used for an adhesive layer included in a conductivereinforcing plate.

Second Embodiment

(Conductive Reinforcing Plate)

The conductive adhesive of the present disclosure can be used to attacha conductive (metal) reinforcing plate to a flexible printed wiringboard.

If a conductive reinforcing plate is to be attached to a flexibleprinted wiring board, a flexible printed wiring board 104 and aconductive reinforcing plate 135 having one surface provided with aconductive adhesive layer 130 made of a conductive adhesive of thisembodiment are first prepared as shown in FIG. 6.

In a method for providing the conductive adhesive layer 130 on thesurface of the conductive reinforcing plate 135, a release substrate (aseparate film) 152 is first coated with a conductive adhesive to form aconductive adhesive film 153 including the conductive adhesive layer 130as shown in FIG. 7, for example. Next, the conductive adhesive film 153and the conductive reinforcing plate 135 are pressed against, andbrought into close contact with, each other. As a result, the conductivereinforcing plate 135 including the conductive adhesive layer 130 asshown in FIG. 8 can be obtained. The release substrate 152 merely needsto be separated before use.

It is possible to use, as the release substrate 152, a base film, madeof polyethylene terephthalate, polyethylene naphthalate, or othermaterials, to which a silicone or non-silicone release agent is appliedonto a surface where the conductive adhesive layer 130 is formed. Thethickness of the release substrate 152 is not specifically limited, andcan be appropriately determined in view of ease of use.

The conductive adhesive layer 130 preferably has a thickness of 15 μm to100 μm. Setting the thickness to be 15 μm or more allows the conductiveadhesive layer 130 to have sufficient filling performance and to beadequately connected to a ground circuit. Further, setting the thicknessto be 100 μm or less can satisfy the need to reduce the layer thickness,and helps reduce cost.

The printed wiring board 104 includes, for example, a base member 142and an insulating film 141 bonded to the base member 142 through anadhesive layer 143. The insulating film 141 has an opening 148 throughwhich a ground circuit 145 is exposed. A surface layer 146 that is agold plated layer is provided on an exposed portion of the groundcircuit 145. Note that the flexible printed wiring board 104 may bereplaced with a rigid substrate.

Next, the conductive reinforcing plate 135 is arranged on the printedwiring board 104 so that the conductive adhesive layer 130 is locatedover the opening 148. Then, the conductive reinforcing plate 135 and theprinted wiring board 104 are vertically sandwiched between two hotplates (not shown) heated to a predetermined temperature (for example,120° C.) so as to be pressed at a predetermined pressure (for example,0.5 MPa) for a short time (for example, 5 seconds). As a result, theconductive reinforcing plate 135 is temporarily fastened to the printedwiring board 104.

Subsequently, the temperatures of two hot plates are set at apredetermined temperature (for example, 170° C.) higher than thoseduring the temporary fixing process. A predetermined pressure (forexample, 3 MPa) is applied to the conductive reinforcing plate 135 for apredetermined time (for example, 30 minutes) using these hot plates.Thus, the conductive reinforcing plate 135 can be fixed to the printedwiring board 104 with the opening 148 filled with the conductiveadhesive layer 130.

Thereafter, a solder reflow process for component mounting is performed.In the reflow process, the flexible printed wiring board 104 is exposedto a high temperature of about 260° C. Non-limiting examples ofcomponents to be mounted include, in addition to a connector and anintegrated circuit, a chip component such as a resistor and a capacitor.If the conductive reinforcing plate 135 is provided on one surface ofthe base member, an electronic component may be disposed on a surface ofthe base member 142 remote from the conductive reinforcing plate 135 tocorrespond to the conductive reinforcing plate 135. However, theconductive reinforcing plate 135 may be provided on both surfaces of thebase member 142.

The conductive adhesive of this embodiment, which has high fillingperformance, sufficiently fills the opening 148 even if the diameter ofthe opening 148 is small, such as 1 mm or less. This can ensure goodconnection between the conductive reinforcing plate 135 and the groundcircuit 145. If the conductive adhesive insufficiently fills theopening, fine bubbles enter the gap between a combination of the surfacelayer 146 and the insulating film 141 and the conductive adhesive layer130. As a result, when the conductive adhesive layer 130 is exposed tohigh temperatures in the reflow process, the bubbles may grow, and theinterconnect resistance of the conductive adhesive layer 130 mayincrease. This may cause the conductive adhesive layer 130 to beseparated at worst. However, the conductive adhesive layer 130 of thisembodiment has good adhesiveness with the surface layer 146 and theinsulating film 141, and keeps having good adhesiveness even after thereflow process. Thus, low interconnect resistance can be maintained.

It is recommended that the interconnect resistance after the reflowprocess as an index of filling performance be as low as possible.However, the interconnect resistance to an opening with a diameter of0.5 mm to be described in examples, for example, can be set to bepreferably 300 mΩ/hole or less, more preferably 250 mΩ/hole or less, andstill more preferably 200 mΩ/hole or less.

The conductive reinforcing plate 135 may be formed of an electricallyconductive material having an appropriate strength. Examples of theelectrically conductive material include nickel, copper, silver, tin,gold, palladium, aluminum, chromium, titanium, and zinc. In particular,stainless steel is recommended to increase the corrosion resistance andstrength.

The thickness of the conductive reinforcing plate 135 is notspecifically limited, but may be preferably 0.05 mm or more and morepreferably 0.1 mm or more, and may be preferably 1.0 mm or less and morepreferably 0.3 mm or less for the purpose of reinforcement.

In addition, a nickel layer is preferably formed on the surface of theconductive reinforcing plate 135. The nickel layer may be formed by anyprocess, but may be formed by electroless plating, electrolytic plating,or any other process. The formation of the nickel layer can improveclose contact between the conductive reinforcing plate and theconductive adhesive.

Also if the conductive adhesive of the present disclosure is used tobond a conductive reinforcing plate, the conductive adhesive preferablyhas the same loss modulus as that obtained if it is used to bond anelectromagnetic-wave shielding film, in order to allow the conductiveadhesive to have sufficient filling performance. To prevent theconductive reinforcing plate from being separated, the conductiveadhesive preferably has the same peel strength as that obtained if it isused to bond the electromagnetic wave shielding film.

EXAMPLES

The conductive adhesive of the present disclosure will now be describedin more detail with reference to examples. The following examples areillustrative, and are not intended to limit the present invention.

<Formation of Electromagnetic-Wave Shielding Film>

A release film was coated with an epoxy resin with a thickness of 6 μmand dried to form a protective layer. Next, predetermined materials weremixed and stirred using a planetary mixer/deaerator to form a conductiveadhesive composition of each of first to third examples and first tothird comparative examples having the composition shown in Table 1.

A value used as the median size of a filling-performance improver wasmeasured with a diffraction particle size distribution analyzer(MICROTRAC 53500, manufactured by Microtrac Inc.) in a volumedistribution mode, where pure water (refractive index=1.33) was used asa solvent, and the refractive index of inorganic particles was set to be1.51.

Next, a paste-like conductive adhesive composition was applied onto theprotective layer, and dried at 100° C. for three minutes to form anelectromagnetic-wave shielding film. The thickness of theelectromagnetic-wave shielding film was 17 μm before pressing and 10 μmafter pressing. The thickness of the electromagnetic-wave shielding filmwas measured with a micrometer.

<Measurement of Loss Modulus>

The dynamic viscoelasticity of the conductive adhesive composition ofeach of the examples and comparative examples was measured with arheometer (MCR302, manufactured by Anton Paar GmbH) in a range of 30° C.to 200° C., and the loss modulus (G″) at 200° C. was determined. As ameasurement sample, a conductive adhesive composition formed into a dischaving a diameter of 25 mm and a thickness of 1 mm was used, and wasmeasured under the following conditions.

-   Plate: D-PP25/AL/S07 Diameter 25 mm-   Angular Deflection: 0.1%-   Frequency: 1 Hz-   Measurement Range: 30 to 200° C.-   Temperature Rising Speed: 6° C./min

<Adhesiveness to Polyimide>

A 180-degree peeling test was performed to measure the adhesivenessbetween polyimide and the conductive adhesive. Specifically, a surfaceof an electromagnetic-wave shielding film near a conductive adhesivelayer was bonded to a polyimide film (Kapton 100EN (registeredtrademark) manufactured by DU PONT-TORAY CO., LTD.) under the conditionsof a temperature of 170° C., a period of three minutes, and a pressureof 2 MPa. Next, a bonding film (manufactured by Arisawa ManufacturingCo., Ltd.) was bonded to a surface of the electromagnetic-wave shieldingfilm near a protective layer under the conditions of a temperature of120° C., a period of five seconds, and a pressure of 0.5 MPa. Next, apolyimide film (Kapton 100EN (registered trademark) manufactured by DUPONT-TORAY CO., LTD.) was bonded onto the bonding film to form a stackfor evaluation. Then, the stack of the polyimide film, the bonding film,and the shielding film was peeled from the polyimide film at 50 mm/min.Table 1 shows average values, where the number n of tests is set to befive.

<Adhesiveness to Gold Plated Layer>

A 180-degree peeling test was performed to measure the adhesivenessbetween a gold plated layer formed on a surface of a copper foil of acopper-clad laminate and a conductive adhesive. Specifically, a surfaceof an electromagnetic-wave shielding film near a conductive adhesivelayer is bonded to the gold plated layer formed on the surface of thecopper foil of the copper-clad laminate film under the conditions of atemperature of 170° C., a period of three minutes, and a pressure of 2MPa. Next, a bonding film (manufactured by Arisawa Manufacturing Co.,Ltd.) was bonded to a surface of the electromagnetic-wave shielding filmnear a protective layer under the conditions of a temperature of 120°C., a period of five seconds, and a pressure of 0.5 MPa. Next, apolyimide film (Kapton 100H (registered trademark) manufactured by DUPONT-TORAY CO., LTD.) was bonded onto the bonding film to form a stackfor evaluation. Then, the stack of the polyimide film, the bonding film,and the shielding film was peeled from the polyimide film at 50 mm/minTable 1 shows average values, where the number n of tests is set to befive.

<Forming a Shield Printed Wiring Board>

Next, the electromagnetic-wave shielding film and the printed wiringboard formed in each of the examples and the comparative examples arebonded together using a press machine under the conditions of atemperature of 170° C., a period of three minutes, and a pressure of 2to 3 MPa, thereby forming a shield printed wiring board.

The printed wiring board used herein included a base member 122configured as a polyimide film, a copper foil pattern 125 simulating aground circuit, an insulative adhesive layer 123, and a cover lay(insulating film) 121 configured as a polyimide film. As shown in FIG.4, the copper foil pattern 125 was formed on the base member 122, andthe insulative adhesive layer 123 and the cover lay 121 were formed onthe copper foil pattern 125. A gold plated layer is provided, as asurface layer 126, on the surface of the copper foil pattern 125. Thecover lay 121 had an opening 128 with a diameter of 0.5 mm to imitate aconnecting portion with the ground.

<Measurement of Interconnect Resistance>

The connectivity between the copper foil pattern 125 and theelectromagnetic-wave shielding film 101 of each of the shield printedwiring boards formed in the examples and the comparative examples wasevaluated by measurement, using a resistance meter 205, of an electricalresistance between two rows of the copper foil pattern 125 having asurface provided with the surface layer 126 that is the gold platedlayer, as illustrated in FIG. 9.

Next, the thus obtained shield wiring boards in the examples and thecomparative examples were made to pass through a hot-air reflowapparatus five times. Thereafter, the interconnect resistance after thereflow process was measured by the technique described above. Bearing areflow process using lead-free solders in mind, a temperature profile,as a condition for the reflow process, was set such that the polyimidefilm of the shield printed wiring board was exposed to a temperature of265° C. for five seconds.

First Example

A solid epoxy resin having an epoxy equivalent of 700 g/eq and a numberaverage molecular weight of 1,200 was used as the first resin component(A1). A polyurethane-modified polyester resin having, an acid value of20 mgKOH/g, a glass transition temperature of 40° C., and a numberaverage molecular weight of 18,000 was used as the second resincomponent. The ratio (A1/(A1+A2)) of the mass of the first resincomponent (A1) to the total mass of the first resin component (A1) andthe second resin component (A2) was set to be 5% by mass. Six parts bymass of 2-phenyl-1H-imidazole-4, 5-dimethanol (2PHZPW, manufactured bySHIKOKU CHEMICALS CORPORATION) was added, as the curing agent (A3), to100 parts by mass of the first resin component (A1).

A dendritic silver-coated copper powder (D-1) having an average particlesize of 13 μm and a silver coverage of 8% by mass was used as theconductive filler (B). The content of the conductive filler with respectto 100 parts by mass of the thermosetting resin (A) was set to be 55parts by mass.

An aluminum salt of tris(diethylphosphinic acid) (OP 935, manufacturedby Clariant (Japan) K.K.) was used as the loss modulus modifier (C). Thecontent of the aluminum salt with respect to 100 parts by mass of thethermosetting resin (A) was set to be 71 parts by mass.

The peel strength of the obtained conductive adhesive from polyimide was5.3 N per 10 mm, and the peel strength of the conductive adhesive fromthe gold plated layer was 8.5 N per 10 mm. If the hole diameter was 0.8mmφ, the initial interconnect resistance was 145 mΩ/hole, and theinterconnect resistance after the reflow process was 190 mΩ/hole. Theloss modulus at 200° C. was 1.3×10⁵ Pa.

Second Example

A second example was similar to the first example except that 71 partsby mass of melamine cyanurate having an average particle size of 2 μmwas used as the loss modulus modifier (C).

The peel strength of the obtained conductive adhesive from polyimide was4.1 N per 10 mm, and the peel strength of the conductive adhesive fromthe gold plated layer was 7.8 N per 10 mm. If the hole diameter was 0.8mmφ), the initial interconnect resistance was 150 mΩ/hole, and theinterconnect resistance after the reflow process was 220 mΩ/hole. Theloss modulus at 200° C. was 1.9×10⁵ Pa.

Third Example

A third example was similar to the first example except that 71 parts bymass of melamine polyphosphate having an average particle size of 6 μmwas used as the loss modulus modifier (C).

The peel strength of the obtained conductive adhesive from polyimide was6.4 N per 10 mm, and the peel strength of the conductive adhesive fromthe gold plated layer was 9.0 N per 10 mm. If the hole diameter is 0.8mmφ, the initial interconnect resistance was 150 Ω/hole, and theinterconnect resistance after the reflow process was 223 Ω/hole. Theloss modulus at 200° C. was 1.4×10⁵ Pa.

First Comparative Example

A first comparative example is similar to the first example except that35.5 parts by mass of an aluminum salt of tris(diethylphosphinic acid)(OP 935, manufactured by Clariant (Japan) K.K.) was used as the lossmodulus modifier (C).

The peel strength of the obtained conductive adhesive from polyimide was7.3 N per 10 mm, and the peel strength of the conductive adhesive fromthe gold plated layer was 8.3 N per 10 mm. If the hole diameter is 0.8mmφ, the initial interconnect resistance was 227 Ω/hole, and theinterconnect resistance after the reflow process was 457 mΩ/hole. Theloss modulus at 200° C. was 4.7×10⁴ Pa.

Second Comparative Example

A first comparative example is similar to the first example except that142 parts by mass of an aluminum salt of tris(diethylphosphinic acid)(OP 935, manufactured by Clariant (Japan) K.K.) was used as the lossmodulus modifier (C).

The peel strength of the obtained conductive adhesive from polyimide was3.3 N per 10 mm, and the peel strength of the conductive adhesive fromthe gold plated layer was 7.2 N per 10 mm. If the hole diameter is 0.8mφ), the initial interconnect resistance was over a measurement limit,and the interconnect resistance after the reflow process was also OL.The loss modulus at 200° C. was 4.4×10⁵ Pa.

Third Comparative Example

A third comparative example was similar to the first example except thatthe loss modulus modifier (C) was not added to a conductive adhesive ofthe third comparative example.

The peel strength of the obtained conductive adhesive from polyimide was6.7 N per 10 mm, and the peel strength of the conductive adhesive fromthe gold plated layer was 6.4 N per 10 mm. If the hole diameter is 0.8mφ, the initial interconnect resistance was 814 mΩ/hole, and theinterconnect resistance after the reflow process was 1,780 mΩ/hole. Theloss modulus at 200° C. was 4.5×10⁴ Pa.

Table 1 summarizes the composition and characteristics of the conductiveadhesive of each of the examples and comparative examples.

TABLE 1 Com. Com. Ex. 1 Ex. 2 Ex. 3 Com. Ex. 1 Ex. 2 Ex. 3 ThermosettingFirst Resin Parts 5 ← ← ← ← ← Resin (A) Component by (A1) Mass SecondResin Parts 95 ← ← ← ← ← Component by (A2) Mass A1/(A1 + A2) (mass %) 5← ← ← ← ← Curing Agent Parts 6 ← ← ← ← ← (A3) by Mass Conductive ShapeDendrite ← ← ← ← ← Filler (B) Average Particle Size 13 ← ← ← ← ← (μm)Parts by Mass 55 ← ← ← ← ← Loss Modulus Component Aluminum MelamineMelamine Aluminum ← — Modifier (C) Salt of Tris Cyanurate PolyphosphateSalt of Tris (diethyl- (diethyl- phosphinic phosphinic acid) acid) Partsby Mass 71 ← ← 35.5 142 — Characteristics Peel Polyimide 5.3 4.1 6.4 7.33.3 6.7 Strength Gold 8.5 7.8 9 8.3 7.2 6.4 (N/10 mm) Plated LayerInterconnect Initial 145 150 150 227 OL 814 Resistance After 190 220 223457 OL 1780 (mΩ/hole) Reflow Loss Modulus (200° C.) 1.3 × 10⁵ 1.9 × 10⁵1.4 × 10⁵ 4.7 × 10⁴ 4.4 × 10⁵ 4.5 × 10⁴ Loss Modulus (170° C.) 1.2 × 10⁵2.0 × 10⁵ 2.0 × 10⁵ 4.3 × 10⁴ 4.3 × 10⁵ 4.2 × 10⁴ Loss Modulus (140° C.)1.4 × 10⁵ 2.1 × 10⁵ 2.4 × 10⁵ 3.9 × 10⁴ 3.4 × 10⁵ 3.0 × 10⁴ StorageModulus 1.1 × 10⁵ 2.0 × 10⁵ 1.4 × 10⁵ 4.4 × 10⁴ 1.0 × 10⁶ 8.1 × 10⁴(190° C.) Storage Modulus 1.1 × 10⁵ 1.9 × 10⁵ 2.0 × 10⁵ 3.5 × 10⁴ 8.7 ×10⁵ 6.1 × 10⁴ (170° C.) Storage Modulus 1.0 × 10⁵ 1.8 × 10⁵ 3.0 × 10⁵2.7 × 10⁴ 2.5 × 10⁵ 1.9 × 10⁴ (120° C.) Storage Modulus (70° C.) 2.6 ×10⁵ 4.1 × 10⁵ 4.2 × 10⁵ 1.1 × 10⁵ 1.1 × 10⁵ 7.7 × 10⁴

FIG. 10 shows the dependence of the loss modulus of the conductiveadhesive of each of the examples and comparative examples ontemperature. The conductive adhesive of each of the first to thirdexamples keeps having a more suitable loss modulus than the conductiveadhesive of each of the first to third comparative examples, in a rangeof temperatures of 170° C. or higher, i.e., in the temperature rangewhere the conductive adhesive is embedded. FIG. 11 shows the dependenceof the storage modulus of the conductive adhesive of each of theexamples and comparative examples on temperature. The storage moduluscurve of the conductive adhesive of each of the first to third examplesfrom 40° C. to 120° C. does not show a distinct maximum value greaterthan the storage modulus at 40° C.

INDUSTRIAL APPLICABILITY

The conductive adhesive of the present disclosure can have its fillingperformance and its adhesiveness improved through modification offactors affecting the filling performance, and is useful as a conductiveadhesive for use in a flexible printed wiring board, for example.

DESCRIPTION OF REFERENCE CHARACTERS

101 Electromagnetic-Wave Shielding Film

102 Printed Wiring Board

104 Printed Wiring Board

111 Conductive Adhesive Layer

112 Protective Layer

113 Shielding Layer

121 Insulating Film

122 Base Member

123 Adhesive Layer

125 Ground Circuit

126 Surface Layer

128 Opening

130 Conductive Adhesive Layer

135 Metal Reinforcing Plate

141 Insulating Film

142 Base Member

143 Adhesive Layer

145 Ground Circuit

146 Surface Layer

148 Opening

151 Support Substrate

152 Release Substrate

153 Conductive Adhesive Film

205 Resistance Meter

The invention claimed is:
 1. A conductive adhesive comprising: athermosetting resin (A); a conductive filler (B); and a loss modulusmodifier (C), wherein the conductive adhesive has a loss modulus at 200°C. from 5.0×10⁴ Pa to 4.0×10⁵ Pa and a storage modulus curve from 40° C.to 120° C. that does not show a distinct maximum value greater than astorage modulus at 40° C.; the thermosetting resin (A) contains a firstresin component (A1) having a first functional group and a second resincomponent (A2) having a second functional group that reacts with thefirst functional group; and the second resin component (A2) is aurethane-modified polyester resin.
 2. The conductive adhesive of claim1, wherein the conductive adhesive contains from 40 to 140 parts by massof the loss modulus modifier (C) relative to 100 parts by mass of thethermosetting resin (A).
 3. The conductive adhesive of claim 1, whereinthe second resin component (A2) has a glass transition temperature from5° C. to 100° C. and a number average molecular weight from 10,000 to50,000.